A METHOD OF PROVIDING TWO SQUINTED BEAM FEEDS TO A MICROWAVE SENSOR AND A COMPOSITE ELLIPTICAL FEED

Abstract

This invention relates development a method and a elliptical multimode feeds, which are laterally displaced in the focal plane of a parabolic reflector of circular aperture in order to achieve two squinted elliptical beams with the required angular spacing. The asymmetry in the secondary beams is realized by illuminating the reflector with elliptic patterns of the elliptical feeds. The elliptical feeds yield different edge illumination tapers in the principal planes. The elliptical feeds consist of elliptical rings similar to the concept of circular coaxial feeds. The parameters of the feeds have been optimized to get the required amplitude and phase distribution in the dominant and higher order modes to synthesize sector shape elliptical radiation patterns. Fig 6;

Full Text

Field of invention
This invention relates to microwave communication, and more specifically relates to development of new type of feed for a reflector antenna.
Background of the invention
Scatterometers are satellite remote sensors to determine the wind direction and speed over water. Scatterometers can provide a wealth of wind velocity observations over the earth's bodies of water. These wind observations have a wind variety of applications including weather forecasting, marine safety, commercial fishing, El Nino prediction and monitoring, and long term climate studies.
All scatterometers are active microwave sensors; they send out a signal and measure how much of that signal retums after interacting with the target. The wind direction is found by determining the angle that is most likely to be consistent the backscatter observed from multiple angles
A microwave sensor called pencil beam scanning scatterometer is used operating at Ku-band. This sensor uses a prime focal reflector antenna. The reflector antenna is illuminated with two feeds lying in the focal plane of the reflector. When the microwave radiation from the two feeds is incident on the reflector, two high gain beams are generated. When the reflector antenna along with feeds is rotated around an axis, the two beams also rotate. When these two scanned beams fall on the ocean surface from a remote sensing satellite on which the reflector antenna is mounted, scattering of beam radiations takes place and some energy is scattered back towards the satellite over which the antenna is mounted. The received radiations from ocean surface are processes to get back ocean wind vectors i.e. speed and direction.

Microwave sensors like pencil beam scanning scatterometer and altimeter require high gain parabolic reflector antennas with feeds at their focus. Compact feeds were required which at Ku-band can illuminate a parabolic reflector antenna, the antenna having diameter of 1.0 meter and f/D ratio of 0.4. Moreover, two high gain inner and outer conically scanned beams with specified angular spacing from the reflector axis was required by the scatterometer.
The following figure shows the schematic of an antenna system where the present invention i.e., elliptical feeds will be used to illuminate a parabolic reflector antenna. In this schematic a pencil beam scanning scatterometer antenna mounted on a satellite deck is shown. This antenna system consists of a parabolic reflector and the two elliptical feeds. The feeds and reflector will rotate around the scanning axis (rotation axis). The present invention discusses only the two feeds of the reflector antenna system of the scatterometer sensor.
Two squinted beams can be achieved by putting two laterally displaced feeds in the focal plane of the reflector. However there is a displacement of feeds that results in the raising of side lobe levels and simultaneously reduction in gain due to beam squint loss. This raising in side lobe levels should be overcome by designing elliptical feeds with larger amplitude taper in the plane of displacement (offset plane) of feeds. The reduction in gain due to scan loss can be compensated by designing the feeds to yield sector shape primary pattems which enhances the spillover and illumination efficiency of the reflector antenna. Sector shape pattem should be achieved with multi-mode feeds where power is distributed in several modes at the feed aperture in such a way that resultant far field pattems become sector shape.
The gain of the reflector antenna can be increased by using multi-mode feeds yielding sector shape pattems such as dual hybrid mode corrugated feeds and multi-ring circular coaxial feeds. But these feeds yield symmetrical sectoral pattems and almost

symmetrical secondary radiation patterns for the required offset of the feeds in the focal plane. To meet the requirements of asymmetric secondary beams, generally elliptical reflectors illuminated by rectangular or elliptical waveguide feeds are used. But the radiation patterns of the available rectangular or elliptical waveguide feeds are not sector shape as required to uniformly illuminate the reflector to enhance the antenna gain. Moreover, the size of the elliptic reflector with these feeds has to be increased to meet the required gain and beam widths, which was not permitted due to the fixed size of the reflector on satellite deck. This necessitated the development of a new multimode elliptical feed for parabolic reflector to achieve elliptical beams with the required gain and beam asymmetry.
Summary of the Invention
The present invention describes a method of providing two squinted beams feeds to a microwave sensor, by providing two laterally displaced feeds at the focal plane of the reflector, linearly polarizing the feeds in the vertical and horizontal plane, providing a higher amplitude taper in the plane of displacement, introducing asymmetry in the circular coaxial aperture and aperture dimensions and optimizing choke size and choke depth of the elliptical feed to get maximum coupling in the higher order modes
The present invention also describes a composite feed operating at Ku-band comprising of two single beams with center frequency of operation at 13.515 GHz, having two elliptical feeds having outer radiating apertures of elliptical coaxial cross section and input channels of rectangular waveguide(WR62) cross section, both being fabricated as a single unit separated by a distance of 56.5mm from each other, a rectangular to elliptical waveguide transitions at the input of feeds to convert TE10 mode of a rectangular waveguide at feed input to the TE11 mode of the elliptical waveguide, wherein the rectangular waveguide being oriented orthogonally at the input of feeds to excite orthogonal linear polarization of microwave radiation and feed radiating apertures with

two concentric rings of elliptical cross section to excite higher order modes to get sector shape far field radiation patterns with different amplitude tapers in the principal planes.
In the present invention describes a new type of elliptical multimode feeds which have been designed to yield different edge illumination tapers in the principal planes being compact and lightweight.
The feeds have to be displaced laterally in the focal plane of a circular parabolic reflector in order to get two squinted inner and outer secondary beams. Since, the lateral offset of the feed increases the side lobe level of the secondary pattern, the feeds have been designed to provide larger edge taper in the offset plane so that even after their displacement, secondary beam side lobe requirement is met. The feed yielding the inner beam is horizontally polarized and the feed yielding the outer beam is vertically polarized. The elliptical feeds presented in this invention consists of elliptical rings similar to the concept of circular coaxial feeds. The elliptical feeds were modeled on the high frequency structure simulator based on the finite element method (FEM). The parameters of the feeds have been optimized to get the required amplitude and phase distribution in the dominant and higher order modes to synthesize the sector shape elliptical radiation pattems
Brief description of drawing
Fig 1 ; A broad view of the scatterometer where this present invention is incorporated
Fig. 2(a):A perspective three-dimensional top view of the two elliptical feeds
Fig. 2(b): A perspective three-dimensional side view of the two elliptical feeds
Fig. 3 : Shows two dimensional sectional view of the elliptical feeds.
Fig 4(a): Shows the front view of the elliptical feeds.
Fig 4(b): Shows rear view of the front view of the elliptical feeds
Fig 4(c): Shows the cross sectional side view of feed assembly

Fig, 5(a): Shows perspective front view of feeds
Fig 5(b): Shows rear view of this integrated assembly of feeds.
Fig. 6 : Shows a the actual pictorial view of the hardware of integrated assembly
Description:
The two elliptical feeds are linearly polarized with one feed having vertical polarization while the other is horizontally polarized. A reflector antenna has been designed in Ku-band at 13.515 GHz to yield the inner and outer secondary beams spacing of ±3.38° from the reflector axis, secondary gain of 40.0 dBi, 3-dB beam widths of L47° and 1.67° in the principal planes respectively and cross-polarization level of-20 dB. The focal length to diameter (F/D) ratio of the circular parabolic reflector is selected as 0.4, requiring ± 64° reflector edge illumination angle. Corresponding to inner and outer beams of scanning reflector antenna for pencil beam scatterometer, the amplitude taper of the radiation patterns of both the elliptical feeds mentioned above should be of the order of-10 dB in one plane and -16 dB in the plane of the offset of feeds respectively, at the reflector edges in order to achieve different 3-dB secondary beam widths in principal planes for the squinted beams respectively. The feeds have been realized by introducing asymmetry in the circular coaxial aperture and aperture dimensions, choke size and choke depth of the elliptical feed have been optimized to get maximum coupling in the higher order modes in order to achieve the required pattem asymmetry, sector shape radiation pattem, VSWR and cross polarization levels. Shaped sectoral radiation patterns of feeds have been achieved by exciting TE11, TM11, TE12 and TM12 modes at feed aperture.
The feeds have been designed and developed in a very compact size by using compact elliptical to rectangular waveguide transitions to convert the elliptical waveguide mode at the feed aperture to the rectangular waveguide mode at input where the feeds are excited.

Fig 1 shows a schematic of pencil beam scanning scatterometer antenna with an elliptical feeds and reflector. The feed in discussed is developed by the present invention.
Fig. 2 shows three dimensional view of the two elliptical feeds along with the interface plate for integrating feeds with a bracket on which reflector spars will be mounted. In Fig. 2 (a), the parts shown as '1' are the inputs of the two elliptical feeds which are fed with WR62 waveguide. The parts shown as '2' are radiating apertures of elliptical feeds. The radiating aperture is realized with coaxial rings of elliptical cross-section as shown in Fig. 2(a).
In Fig. 2(b), the parts shown as '3' are tapered transitions from rectangular wavguide cross-section at input of feeds to the elliptical cross section of feeds behind aperture. The part shown as '4' in Fig. 2(b), is the feed interface with a bracket which will hold these two feeds and in tum theis bracket will be the interface between feed assembly and the three spars mounted with reflector of diameter 1 meter.
Fig. 3 shows sectional view of the elliptical feeds. In Fig 3, '1' is the rectangular waveguide (WR62) inputs of feed-1, '2' is rectangular waveguide(WR62) inputs of feed-2, *3* is the elliptical to rectangular waveguide transitions and *4* is the elliptical coaxial apertures of feeds
Fig. 4 shows different two dimensional views of the elliptical feeds. Fig 3(a) or *r shows the front view of the elliptical feeds. Fig 4(b) or '2' shows back view of the front view of the elliptical feeds and Fig 4(c) or '3' shows the sectinal side view of feed assembly.
Fig. 5 shows front view of feeds with feed bracket and back view of feeds with feed bracket. Fig 5(a) or '1' shows front view of the integrated assembly of feeds,

interface plate and bracket where feeds and reflector antenna spars will be mounted. Fig 5(b) or '2' shows back view of this integrated assembly of feeds, interface plate and bracket.
Fig. 6 shows the photograph of the developed hardware of integrated assembly including two elliptical feeds, interface plate between feed and bracket and the mounting bracket for holding feeds and three spars of a parabolic reflector.
Keeping these feeds (separated by 56.5 mm) in the focal plane of a parabolic reflector of projected diameter of 1 meter, measured secondary gain value of 40 dBi has been achieved. Apart from this the required elliptic beamwidth, side lobe level and cross-polar radiation, beam separation has been achieved in the measured secondary pattems of the parabolic reflector illuminated by these elliptical feeds.
The measured and designed specification is given in the table below.

The invention is particularly useful as feeds for reflector antennas for space and ground applications. The present hardware has been mainly developed for a pencil beam scanning scatteromter antenna for space borne remote sensing application of ocean wind

vector retrieval. The two feeds, which are orthogonally polarized, are kept symmetrically in the focal plane of the parabolic reflector in order to achieve two squinted high gain pencil beams with the required angular spacing of 6.8 degree. The elliptical feeds have features of shaped sectoral radiation pattems, low cross-polarization, low reflection loss, low weight, compact design, low insertion loss, single piece fabrication of two feeds and orthogonal polarization of two feeds.

We Claim
1. A method of providing two squinted beams feeds to a microwave sensor, comprising
the steps of;
providing two laterally displaced feeds at the focal plane of the reflector
linearly polarizing the feeds in the vertical and horizontal plane;
providing a higher amplitude taper in the plane of displacement;
introducing asymmetry in the circular coaxial aperture and aperture dimensions; and
optimizing choke size and choke depth of the elliptical feed to get maximum coupling in the higher order modes
2. A method as claimed in claim 1, wherein the step of linearly polarizing the feed is by horizontally polarizing the feed yielding the inner beam and vertically polarizing the feed yielding the outer beam.
3. A method as claimed in claim 1, wherein the step optimizing is to achieve the required pattern asymmetry, sector shape radiation pattern, VSWR and cross polarization levels.
4. A composite elliptical feed operating at Ku-band comprising of two single beams with center frequency of operation at 13.515 GHz, the feed comprising;
two elliptical feeds having outer radiating apertures of elliptical coaxial cross section and input channels of rectangular waveguide(WR62) cross section, both being fabricated as a single unit separated by a distance of 56.5mm from each other;
rectangular to elliptical waveguide transitions at the input of feeds to convert TE10 mode of a rectangular waveguide at feed input to the TE11 mode of the elliptical waveguide;
wherein the rectangular waveguide being oriented orthogonally at the input of feeds to excite orthogonal linear polarization of microwave radiation; and

feed radiating apertures with two concentric rings of elliptical cross section to excite higher order modes to get sector shape far field radiation patterns with different amplitude tapers in the principal planes;
5. The composite feed as claimed in claim 4, wherein the said feed is connected to the antenna by means of a bracket and the said brackets are connected to the feed by means of an interface,
6. The composite feed as claimed in claim 4, wherein the amplitude taper is -10 dB in one feed and -16 dB in the offset feed.
7. The composite feed as claimed in claim 4, wherein the feed yielding the inner beam is horizontally polarized and the feed yielding the outer beam is vertically polarized.
8. A pencil beam scanning scatterometer antenna having an elliptical feed as described in any of the preceding claims.
9. A method of providing two squinted beams feeds to a microwave sensor as herein before described in the specification and with reference to figures 2- 6.
10. A composite feed operating at Ku-band comprising of two single beams with center frequency of operation at 13.515 GHz as herein before described in the specification and with reference to figures 2- 6.
Dated this 26th day of September 2007